1. Field of the Disclosure
This disclosure relates to methods, apparatus and software for modelling the behaviour of materials which are crushed particularly, but not exclusively, in the context of composite vehicle body parts under impact.
2. Discussion of the Background Art
Fibre-reinforced composite materials, particularly carbon fibre composites are becoming an increasingly important part of the designs of new cars and other vehicles. Composites are very light compared to their metal equivalents, even aluminium, and can be formed into complex shapes that can do the same job as many welded metal stampings. Composites also have the ability to absorb high amounts of energy during impacts which make them ideal for automotive, aerospace, rail or civil applications. For example, whereas steel can only absorb up to 20 kilojoules per kilogramme and aluminium approximately 30 kilojoules per kilogramme, carbon composites can absorb up to 80 kilojoules per kilogramme.
Part of the reason that composites have such good energy absorption characteristics is that in addition to the traditional failure modes, such as bending and cracking, through which metals can fail, composites exhibit a completely separate failure mode not present in metals. This is known in the art as the crush failure mode. In this mode the crushed material has essentially no residual strength after it has absorbed the energy. Instead, the composite material is transformed into very small pieces of debris and loosely connected fibres after it has been crushed. This also means that less space is required than in an equivalent metal structure. This is because in a metal structure space must be provided in designated crumple zones to accommodate the buckled metal.
On the microscopic scale such materials absorb energy by local disintegration of the material, by matrix cracking, fibre buckling and fracture, frictional heating etc. Viewed on a macro scale, the material is essentially crushed or consumed by the impact on a continuous basis, and the originally consolidated material is turned into a non-structural debris.
The Applicant's earlier publication WO 2006/003438 describes a revolutionary technique for allowing materials which exhibit a crush failure mode as outlined above to be modelled accurately. This technique, known as CZone™ is now being widely used to design new parts for automobiles and other vehicles.
Prior to CZone, existing finite element analysis techniques could only deal with elements of composite material by existing failure modes and so treated the whole element or the individual layers together making up a laminate, as maintaining their integrity until the ‘classic’ failure stress value is reached, whereafter the element or each layer at a time is simply deleted from the analysis. This approach gives inaccurate results as it does not reflect how these materials behave in practice. The material does not suddenly disappear but is gradually ‘consumed’ as it turns to fine debris at the impacting surface. This approach also leads to another problem that arises from the very large forces generated in the model (which do not in fact arise in the actual material being modelled) from failing elements using the classical failure stress. These forces are propagated through the model and can cause unexpected (and unrealistic) failures in the back-up structure.
Despite the usefulness of the CZone technique, the Applicant has now identified that there are some situations where the basic technique is not as accurate and has devised a revised technique which aims to address this.